1. Field of the Invention
The present invention relates to coking devices and processes and more particularly to such devices and processes which automatically control the hydrocarbon flow rate and process temperatures therein.
2. Description of Prior Art
In delayed coking processes, heavy petroleum hydrocarbons such as tar, asphalt or the like are circulated through a furnace for heating to coking temperature and then delivered to a coke drum where a coke product forms from the heated heavy hydrocarbons. The overhead of the coke drum is quenched and circulated to a combination tower for separation of the overhead into gas oil, fuel oil, naptha and the like. When the coke drum is filled with coke, a mechanical process of removing the coke from the coke drum is begun. Generally, during the process of removing the coke from the coke drum, the circulation of the heated heavy hydrocarbon is switched to a second coke drum which fills with coke as the first coke drum is being mechanically emptied. The time between switching of the flow between coke drums is the coking cycle time.
One problem with the coking processes presently known is that it is difficult to control the rate of coking to produce a maximum amount of coke in a minimum cycle time. This is because the amount of coke produced by different feeds varies greatly. Also, the hydrocarbon feed has significant differences in its heat transfer characteristics. Fouling of tubing carrying the hydrocarbon feed further changes the heat transfer characteristics. This results in non-uniform heating of the feed material which, in turn, changes the coking characteristics of the feed material. Accordingly, the furnace heating capacity, the coke drum size, and the cycle times are over designed producing inefficiences in each area.
In most coking processes the temperatures, cycle times, feed flow rates and other parameters are manually controlled. The experience of the operator is the only way in which the process is made more efficient. However, an improvement over this manual operation is shown in U.S. Pat. No. 4,176,052. In this process, a radiation source and detector are disposed at a predetermined level of each coke drum and the amount of time to fill the coke drum to this level allows a prediction of a desired rate to fill the remainder of the coke drum. By using this prediction together with the information of feed rate from the last coke cycle, the coking process can be more efficiently operated.
While the prediction of coking based on the previous cycle and a portion of the present cycle improves the control of the coking process, it is still relatively inefficient. Particularly, it does not always produce a maximum coking rate and does not improve the heating inefficiences which are inherent in the present coking processes.